California’s San Joaquin Valley is one of the most productive agricultural regions in the country. It is also where, among the fresh fruits and vegetables, an estimated half-a-million plants of marijuana are growing openly, some allegedly being sold for non-medicinal purposes.

“In the last two years, we’ve seen large-scale commercial farming operations of marijuana and explosions of backyard marijuana grows,” said Lieutenant Rick Ko of the Fresno County Sheriff’s Office. Ko says the spike in cannabis cultivation is a result of California’s booming medical marijuana trade.

California was the first state to legalize medical marijuana almost 16 years ago to help people manage the nausea and pain associated with serious illnesses such as cancer and HIV.

Law enforcement authorities say traffickers are hiding behind California’s medical marijuana laws and distributing the drug illegally. Although medical marijuana is prohibited under federal law, 16 states and the District of Columbia have legalized use and enforcement of federal law is often limited to the worst offenders.

In California, Lt. Ko’s team is attempting to limit the abuse of state and local medical marijuana regulations.

“We’re looking for the people that are diverting medical marijuana from legitimate uses to either organized crime or profiteering activities,” Ko said.

Some patients in California are getting doctors’ recommendations to grow up to 99 plants of medical marijuana.

Federal sentencing guidelines are stiff for those caught growing more than 100 plants. However, Lt. Ko says even the lower amount is more than any single patient could ever need for personal use. Cannabis plants cultivated in the San Joaquin Valley can grow up to 18 feet high and yield up to three pounds of medical grade marijuana.

“Let’s just take a conservative estimate of a pound per plant. And if you have 99 pounds then you are looking at over 89,000 doses in a year,” Ko said. “So you would have to smoke a joint every six minutes of every hour of every day for a year to use the 99 plants.”

What’s more, some patients form growing collectives, amassing numerous state-sanctioned medical marijuana cards to grow multi-acre fields of marijuana. Last year, the Fresno County Sheriff’s Office along with federal agents raided a 54-acre farm with more than 25,000 marijuana plants.

“There’s kind of uncontrolled, explosive growth of huge cultivation operations here which are resulting in marijuana being sold in huge amounts being shipped all over the country,” says U.S. Attorney Benjamin Wagner, whose region includes the San Joaquin Valley.

Last fall, U.S. Attorneys in California vowed to work with local authorities to crack down on medical marijuana, targeting growers and dispensaries alike. Last year, Wagner prosecuted traffickers who moved more than half a million dollars worth of marijuana from a Fresno farm all the way to Massachusetts.

The federal crackdown on medical marijuana has sparked critics like Ethan Nadelmann, the Executive Director of the Drug Policy Alliance, an organization based in New York City that advocates for the decriminalization of drugs.

Nadelmann says that the raids on medical marijuana farms and dispensaries are a waste of government resources in a time when, according to recent Gallup polls, fifty percent of Americans are in favor of legalizing marijuana.

“When the gangsters are making billions of dollars off of providing a commodity that tens of millions of Americans want, you make it legal and you tax it and control it and regulate it,” Nadelmann said.

California State Assemblyman Tom Ammiano is sponsoring a bill that would do just that. A long-time proponent of medical marijuana, Ammiano now wants to tax the $14-$15 billion dollar illegal marijuana trade in California.

“We estimated that if marijuana were legalized, it may result in maybe two billion bucks more … in California’s budget,” the Democratic assemblyman representing San Francisco said.

Law enforcement authorities disapprove of efforts to expand marijuana’s legalization. During an interview conducted in a Fresno residential neighborhood, Lt. Ko argued that marijuana cultivation in California has already spiraled out of control.

“Right now, you can hear an ice cream truck. You can hear kids playing. You have, probably, kids next door, kids in this house, school kids walking down the street,” he said. “Right around a marijuana grow.”